-LRB- CNN -RRB- -- U.S. gasoline prices have declined for four weeks straight and now average more than 20 cents a gallon cheaper than a year ago , according to a new nationwide survey .

The average cost of a gallon across the continental United States of regular stands at $ 3.71 , down 3 cents from two weeks earlier , said Trilby Lundberg , publisher of the Lundberg Survey . Prices have fallen nearly 9 cents a gallon in the past four weeks and are 22 cents cheaper than at this point in 2012 , Lundberg said .

Lower prices in the Europe 's benchmark Brent crude oil are largely behind the most recent fall . More U.S. refining capacity coming back on line after seasonal maintenance also contributed , Lundberg said .

`` From there , short-term , we may see more price-cutting soon , perhaps on the order of this approximate 3-cent decline , '' she said . `` But the current picture suggests it wo n't be large . ''

Gas prices broke a three-month upward spiral in early March , which had climbed nearly 54 cents since late December .

The Lundberg Survey canvasses about 2,500 filling stations across the Lower 48 states every two weeks . The most expensive fuel in the latest survey , conducted Friday , was in Chicago , where pump prices averaged $ 4.10 a gallon ; the cheapest could be found in Billings , Montana , at $ 3.33 , Lundberg said .

Average per-gallon prices in other cities :

Atlanta : $ 3.57

Baton Rouge , Louisiana : $ 3.45

Boston : $ 3.75

Denver : $ 3.53

Las Vegas : $ 3.72

Memphis , Tennessee : $ 3.43

Miami : $ 3.74

Philadelphia : $ 3.64

San Francisco : $ 4.07

Seattle : $ 3.81

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Cheaper European crude helps push down U.S. gas prices

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The average price of a gallon of U.S. regular was $ 3.71 on Friday

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The most expensive gas was in Chicago ; the cheapest in Billings , Montana